Ultrahigh vacuum chamber



June 15, 1965 PODLASECK, JR

ULTRAHIGH VACUUM CHAMBER Filed Jan. 15, 1963 TO VACUUM PING IN OUT COOLANT IN OUT HEATING TO ULTRA *HIGH MEDIUM VACUUM PUMPING OR SYSTEM COOLANT STANLEY E. PODLASECK, JR"

IN V EN TOR.

min

A TTORNE Y the pressure gradient thereacross. necessitate the use of a high capacity pumping system United States Patent Office 3,189,211 Patented June 15, 1965 3,189,211 ULTRAHIGH VACUUM CHAMBE Stanley E. Podlaseclr, Jr., Edgewood, Md., assignor to Martin-Marietta Corporation, New York, N.Y., a corporation of Maryland Filed Jan. 15, 1963, Ser. No. 251,563

9 Claims. (Cl. 2209) The invention relates to vacuum environments and, more particularly, to a method and an apparatus for creating ultrahigh vacuum conditions.

The advent of the space age has created an urgent need for ultrahigh vacuum environments wherein various equipments can be tested and evaluated under conditions which may be expected to be encountered during the exploration of space. Other operations, such as advanced manufacturing techniques, are dependent upon reliable high vacuum operating conditions and are readily enhanced by improvements in methods of and apparatus for creating such environments.

Conventional chamber structures used in conjunction with mechanical type vacuum pumps are capable of producing vacuums on the order of torr while diffusion, ion or cryogenic pumping systems have enabled conventional chamber structures to be evacuated to pressures of approximately 10" torr. These latter referred to types of pumping systems are theoretically capable of producing even higher vacuums, but gas diffusion from and through the walls of conventional chamber structures prevent these often desirable and necessary environments from being obtained and reliably maintained.

It has been previously recognized that gas diffusion from a material is accelerated when the material is subjected to elevated temperatures and retarded when the material is subjected to lower temperatures. Advantage of this situation has been taken heretofore to produce ultrahigh vacuum environments. By initially heating and subsequently cooling the walls of the vacuum chamber enclosure as the chamber is evacuated, others have claimed to have produced vacuums on the order of 10* torr. However, under such circumstances it has been necessary to provide a relatively thick chamber wall in order that it will be able to withstand the large pressure differential eventually established thereacross. The initial gas content of any chamber Wall is directly proportional to its thickness and, in addition, the diffusion of gas through a chamber wall is directly proportional to Both of these facts over a relatively long pump-down cycle and the use of elaborate sealing techniques in order that an ultrahigh vacuum environment may be produced employing techniques heretofore devised. These same facts also place a theoretical limit on the degree of vacuum which may be first produced and then reliably maintained within such chambers.

It is therefore a primary object of this invention to provide a method and an apparatus which greatly facilitate the creation of ultrahigh vacuum environments. An improved method and apparatus are provided which permit higher vacuums to be produced than heretofore possible. In addition, the method and apparatus of this invention permit ultrahigh vacuum environments to be inexpensively and efficiently produced and reliably maintained.

These and other purposes and advantages of this invention will become apparent as the following description is read in connection with the accompanying drawing, which diagrammatically illustrates the process and apparatus of the present invention.

Briefly, this invention comprises forming a vacuum chamber enclosure, the wall structure of which includes a honeycomb panel assembly having a thin inner wall and a thin outer wall joined by a perforated honeycomb core. This vacuum chamber enclosure is mounted on a base in a manner whereby a double 0 ring seal is effected between the bottom of the enclosure wall and the base. Coils capable of alternately carrying a heating medium and a coolant are mounted within the honeycomb panel assembly in the immediate vicinity of and in contact with the inner wall thereof. A second coil, capable of conducting a coolant, is mounted within the honeycomb panel assembly in the immediate vicinity of the double 0 ring seal. The interior of the vacuum chamber enclosure is connected through the aforesaid base on which the enclosure is mounted to an ultrahigh vacuum pumping system, while the interior of the honeycomb panel assembly is connected to a second vacuum pumping system. During the initial stages of evacuating the spaces Within the vacuum chamber enclosure and the interior of the honeycomb panel assembly, the inner wall of the honeycomb panel assembly is heated to facilitate the diffusion of gas therefrom. Simultaneously the double 0 ring seal is cooledto prevent heat damage thereto. During the later stages of evacuation, the inner wall of the honeycomb panel assembly is cooled to prevent further diffusion of any residual gas retained therein into the vacuum chamber. Since there is virtually no pressure differential across the inner wall of the honeycomb panel assembly, gas diffusion through this wall into the vacuum chamber is practically non-existent.

Referring now to the drawing in detail, which diagrammatically illustrates a preferred embodiment of the present invention, there is shown a vacuum chamber enclosure 2 comprising a top member 3 joined to a cylindrical wall member 4. The cylindrical wall member 4 is formed of a honeycomb panel assembly 5 comprising a thin inner wall 6 and a thin outer wall 7 brazed to a honeycomb core 8. Provided in the honeycomb core 8 is a plurality of perforations 9 so that, in effect,each cell of the honeycomb core communicates with its adjacent cells without materially detracting from the integral strength of the cylindrical wall member 4.

Brazed to the lower end of the honeycomb panel assembly 5 is a metal ring 10 provided with a pair of annular recesses 11 and 12 in the bottom. face thereof. The upper face of the metal ring 10 is provided with a groove 13 in which is seated a coiled tube 14 which passes through apertures 15 suitably provided in the honeycomb core 8 and the ends of which pass through apertures provided through the outer wall 7 to connect to the tubes 16 and 17, respectively. The open ends of these tubes 16 and 17 are in turn connected to a refrigerant system i (not shown) such that a coolant can be caused to flow through the coiled tube 14. Further provided in the metal ring 10 is a plurality of recesses 18 annularly spaced therearound and which communicate with a similar number of tubular passages 19 for a purpose to be described in detail hereinafter.

Brazed to the upper end of the honeycomb panel assembly 5 is a metal ring 20 which is provided with an annular recess 21 to receive the top member 3 of the vacuum chamber enclosure 2, the top member being brazed to the ring 20. The top member 3 is similar in construction to the honeycomb panel assembly 5 of the cylindrical wall member 4 being formed of a honeycomb panel assembly 22 comprising a thin inner wall 23 joined to a thin outer wall 24 by a honeycomb core 25. The honeycomb core 25 is provided with a plurality of perforations 26 so that, in effect, each cell thereof communicates with each of its adjacent cells. Annularly spaced around the periphery of the inner wall 23 and passing therethrough is a plurality of apertures 27 which communicate with a similar number of passages 28 annularly provided at spaced intervals through the metal ring such that the interior of the honeycomb panel assembly 22 communicates with the interior of the honeycomb panel assembly 5. An inwardly spiraled coil 29 is located within the honeycomb panel assembly 22 and maintained in contact with the inner wall 23 thereof by being positioned through apertures 3% provided in the honeycomb core 25. Passing through an opening 31 in the outer wall 24 of the honeycomb panel assembly 22 is a tube 32 which is coupled to one end of the spiraled coil 29. Similarly, a tube 33 passes through an opening 34 in the outer wall 24 of the honeycomb panel assembly 22 to connect to the other end of the spiraled coil 29. The open ends of the tubes 32 and 33 are alternately connected to heating and refrigerant systems (not illustrated) for purposes to be subsequently more fully explained.

An upwardly spiraled coil 35 is located Within the honeycomb panel assembly 5 and maintained in contact with the inner wall 6 thereof by being positioned through apertures 36 suitably provided in the honeycomb core 8. One end of the spiraled coil 35 is coupled to the tube 32 by a tube 37 which passes through an aperture provided through the outer wall 7 of the honeycomb panel assembly 5, while the other end thereof is coupled to the tube 33 by a tube 38 which passes through another aperture provided in the outer wall 7.

Additionally provided through the outer wall 7 of the cylindrical wall member 4 is a plurality of openings 39 annularly spaced at 90 intervals therearound and which are coupled to an annular tube 40 by four tubular connectors 41. The annular tube 49 is in turn coupled to a vacuum pumping system (not shown), which may be of the mechanical type, by a tube 42.

A tubular member 43 provided with outwardly depending flanges at each end thereof is mounted in an aperture 44, passing through the honeycomb panel assembly 5, the flanged ends of the tubular member being brazed to the inner wall 6 and the outer Wall 7, respectively, of the honeycomb panel assembly 5. Mounted within and brazed to the tubular member 43 is a second tubular member 45 one end of which is provided with an outwardly depending flange 46. The electrical leads 47 of a nude ionization gauge 48 located inside the vacuum chamber enclosure 2 pass through the tubular member 45 and a glass seal 49 to connect the nude ionization gauge to an ionization gauge controller 50. A metal ring 51, located around and fused to the glass seal 49, seals the tubular member 45 by being brazed to the flange 46 thereof. If necessary or desirable other electrical leads may be passed through either the top member 3 or the cylindrical wall member 4 of the vacuum chamber enclosure 2 in a similar manner. Also, if desirable, a sight glass may be similarly mounted over a similar type opening provided in either the top member 3 or the cylindrical wall member 4 of the vacuum chamber enclosure 2.

The vacuum chamber enclosure 2 is mounted on the upper face 52 of the flanged portion 53 of a flanged coupling 54 which is connected to the battle of any conventional ultrahigh vacuum pumping system (not illustrated) comprising, for instance, a mechanical type vacuum pump coupled to a diffusion, ion or cryogenic type pump. Located within the annular recesses 1i and 12, respectively, of the metal ring 10 are an inner O ring 55 and an outer O ring 56 which together eifect a double 0 ring seal between the vacuum chamber enclosure 2 and the flanged coupling 54. A perforated disk 57 seats on the upper face 52 of the flanged coupling 54 so as to be disposed within the vacuum chamber enclosure 2 and over the tubular passage 58 of the flanged coupling.

In operation, an article to be subjected to an ultrahigh vacuum environment is placed on the perforated disk 57 which is seated on the flanged coupling 54 as illustrated and the vacuum chamber enclosure 2 is seated on the flanged coupling so that the 0 rings 55 and 56 effect a double 0 ring seal therebetween. Both pumping systems are energized to commence the pump-down operations of the space within the vacuum chamber enclosure 2 and the space within honeycomb panel assemblies 5 and 22. It will be noted that gases within the top member 3 are drawn through the apertures 27 and the passages 28 into the honeycomb panel assembly 5 from which gases are withdrawn through the annular tube 40 and the tube 42. A heating medium such as steam is caused to circulate through the spiraled coils 29 and 35 by connecting the open ends of the tubes 32 and 33 to a heating system (not shown) while a coolant such as chilled water is caused to flow through the coil 14 by connecting the open ends of the tubes 16 and 17 to a refrigerant system (not shown). Since the coils 29 and 35 are in immediate contact with the inner walls 23 and 6 of the honeycomb panel assemblies 22 and 5, respectively, these inner walls are readily raised to an elevated temperature to facilitate the diffusion of gas therefrom. The coolant flowing through the coil 14 protects the 0 rings 55 and 56 from damage which they might otherwise incur from an elevated temperature condition.

Since the bottom face of the metal ring 10 is vented to the interior of the honeycomb panel assembly 5 through the plurality of recesses 18 and tubular passages 19, the pressure differential across the inner O ring 55 will subsequently be reduced to an absolute minimum while the pressure differential across the outer O ring 56 will approach one atmosphere. Consequently, any diffusion of gas through the O ring seal, i.e. through the O ring 55, will be withdrawn through the annular tube 40 and the tube 42 to the vacuum pumping system. The net result is the establishment of an extremely effective seal between the vacuum chamber enclosure 2 and the flanged coupling 54. The honeycomb panel assemblies 5 and 22 from which the cylindrical wall member 4 and top member 3 are formed, respectively, comprise extremely rigid integral structures permitting the inner and outer walls thereof to be extremely thin members while still withstanding the large pressure differential that will ultimately exist thereacross. The fact that this type of structure permits the inner walls 6 and 23 and the outer walls 7 and 24 of the vacuum chamber enclosure 2 to be extremely thin results in wall members initially having a relatively low gas content and, consequently, only a short period of time is required for the bulk of this gas content to be withdrawn therefrom when low vacuum conditions are established on both sides of the inner walls particularly when such a condition is maintained at elevated temperatures.

After it has been determined that this operation has been maintained a sufficient period of time to remove the larger portion of the initial gas content of the inner walls 6 and 23 and the outer walls 7 and 24, the tubes 32 and 33 are disconnected from their associated heating system and connected to their associated refrigerant system. If desirable and convenient, the tubes 16 and 17 may be simultaneously disconnected from their associated refrigerant system. Coolant is now directed through the spiraled coils 29 and 35 to lower the temperature of the inner walls 23 and 6 of the honeycomb panel assemblies 22 and 5, respectively. Reducing the temperature of the ITlIlQF WQHS 2.3 and 6 retards to an absolute minimum the diffusion of any residual gas remaining therein into the vacuum chamber and also any diffusion of gas therethrough into the vacuum chamber. The possibility of any gas diffusing completely through the inner walls 22 and 6 of the vacuum chamber enclosure 2 into the vacuum chamber is further eliminated by the fact that there is no pressure gradient of any consequence across these memers.

Since the spiraled coils 29 and 35 are spaced from the outer walls 24 and 7 of the honyecomb panel assemblies 22 and 5, respectively, there is virtually no heat transfer therebetween by conduction. In addition, since a vacuum environment exists within the honeycomb panel assemblies a particular material. material is dependent on the surface characteristics there- 22 and ,there is in effect no heat transfer between the spiraled coils 29 and 35 and the outer walls 24 and '7, respectively, by convection. Virtually any heat transfer between the coils 29 and 35 and the outer. walls 22 and 5, respectively, is limited to that resultingfrom radiaton and, consequently, the inner walls 23 and 6 of the honeycomb panel assemblies 22 and 5, respectively, are most efficiently heated and cooled. N

The pressure within the vacuum chamber enclosure 2 is sensed by the nude ionization gauge 48 and indicated to the operator on the ionization gauge controller 59. The particular structure of the vacuum chamber enclosure 2, the provisions of alternately heating the inner Walls 6 and 23 of the honeycomb panel assemblies 5 and 22, respectively, during the initial stages of evacuation and cooling these inner Walls during the later stages of evacuation, and the method of operation provided permit ultrahigh vacuums to be produced in an extremely eilicient and rapid manner and, in addition, permit higher vacuums to be obtained and reliably maintained than heretofore possible.

Since the pressure differential across the inner walls 6 and 23 of the vacuum chamberenclosureZ is maintained at a minimum, these walls may be formed of materials which are as thin as manufacturing fabrication techniques permit. A practical limitation on the thickness of these ture employed, satisfactory results will normally be obtained using inner and outer walls of the same thickness. The practical limitation on the thickness of the inner walls 6 and 23' in order that ultrahigh vacuum conditions may be produced is approximately .010 inch. Any conventional materials may be used in forming the honeycomb panel assemblies 5 and 22; however, it is desirable to select those materials having a relatively low initial gas content.

Assuming the same thickness, the gas content of a material is not only dependent on the particular type of material considered but also on the method employed in producing In addition, the gas content of a of, i.e., the rougher the surface, the greater adsorption of gas therein. Consequently, while structure formed of other materials, such as aluminum have proved satisfactory for forming honeycomb panel assemblies for these purposes, better results may be expected by employing stainless steel since a highly finished surface may readily be provided thereon.

In most applications the minimum width of the honeycomb cores 8 and 25 of the vacuum chamber enclosure 2 will be determined by the size of the spiraled coils 35 and 29, respectively, mounted therein. No limitation is placed on the maximum or minimum size of the vacuum chamber enclosure that may be employed by the particular chamber construction and sealing method described. However, larger chambers may require that the thickness of the outer walls 7 and 24 be increased, the use of thicker material in forming the honeycomb cores 8 and 25, and/or the attachment of reinforcing rings or bars to the inner walls 6and 23 of the vacuum chamber enclosure 2. Vacuum chambers having large diameters may require the use of a base formed of a honeycomb panel assembly similar in construction to the honeycomb panel assembly 22. Such a base would include an inwardly spiraled coil capable of alternately conducting a heating.

medium and a coolant and would be provided with a metal ring around its outer periphery on which the cylindrical wall member 4 would seat to effect a double 0 ring seal therebetween. An opening would be provided through this honeycomb panel base to connect the interior of the vacuum chamber to an ultrahigh vacuum pumping system and the interior of the honeycomb panel base would communicate with the interior of the cylindrical wall member 4. In such cases there would be no requirement for a disk similar to the perforated disk 57 shown in the drawing.

The perforations 9 and 26 in the honeycomb cores 8 and 25, respectively, should be as numerous and as large as possible without materially affecting the integral strength of the vacuum chamber enclosure 2 in order that the resistance offered to the passage of gases through the honeycomb cores will be at a minimum. For the same reason, i.e., minim-um resistance to the passage of gases through the honeycomb cores 8 and 25, it is desirable that the outer wall 7 of the vacuum chamber enclosure 2 be provided with a plurality of openings 39 communicating with the annular tube 40 and that the inner wall 23 of the honeycomb panel assembly 22 be provided with a plurality of apertures 27 communicating with a similar number of passages 28 through the metal ring 20.

It will be noted that in the particular embodiment of the invention illustrated, the spiraledcoils 29 and 35 alternately carry a heating and a cooling medium. If desirable, two sets of coils, one capable of conducting a heating medium and the other capable of conducting a cool ing medium, may be employed for these purposes. Ad-

jacent turns of the spiraled coil 29 within the honeycomb panel assembly 22 and of the spiraled coil 35 around the inner wall 6 of the honeycomb panel assembly 5 should be spaced such that the inner walls 23 and 6, respectively,

of the vaouum'chamber enclosure 2 are uniformly heated obtained when the pressure differential across the inner walls 6 and 23 of the vacuum chamber enclosure 2 is an absolute zero, from a practical standpoint it has been found that system performance. isnot degraded to any noticeable extent when the pressure within the walls of the vacuum chamber enclosure 2 is in theorder of 10* torr at the time that an ultrahigh vacuum condition exists Within the vacuum chamber itself. In addition, it has been found that the perforations 9, and 26 in the honeycomb cores 8 and 25, respectively, offer a resistance to the .passage of gases therethrough which limits the degreeof vacuum which may be established within the walls of the vacuum chamber enclosure 2. Consequently, if both chambers to be evacuated, i.e., the space Within the walls of the vacuum chamber enclosure 2 and the vacuum chamber itself, were connected to the same ultra-high vacuum pumping system, the vacuum environment within the former space would be improved only an insignificant amount while the vacuum environment within the latter space would be materially. degraded, Therefore, as a or cryogenic type .pump may be employed to evacuate the vacuum charrrberitself and the mechanicalpunrp of this same system may be coupled to the space between the Walls of the vacuum chamber enclosure 2.

The time required to create an ultrahighvacuum environment using the process and apparatus of the present invention will naturally vary from system to system and is dependet upon the size of the vacuum chamber, the materials from which the enclosureis formed, the, types and capacities of the pumping systems employed, and the types of cooling and heating means used. While the structure of the top member 3 and cylindrical wall member 4 of the vacuum chamber enclosure 2 illustrated in the drawing essentially comprise a honeycomb panel assembly, other similar forms of structures which may be used to effect the same desirable results will readily suggest them selves to those skilled in the art. For instance, a corrugated type of core structure may be employed between thin outer and inner walls to form a vacuum chamber enclosure satisfactory for use in many applications. The only requirements are that the inner and outer walls be joined by a core material to form an integral structure capable of supporting the load thereacross while at the same time permitting the walls thereof to be formed of thin materials, that the space between the inner and outer walls be capable of evacuation and that, if desirable, heating and cooling tubes may be located between the two walls of the structure and in the immediate vicinity of the inner wall thereof.

This invention may be performed and/ or embodied in other ways without departing from the spirit or essential character thereof. The process and embodiments of the invention described herein are therefore to be considered as in all respects illustrative and not restrictive, the scope 'of the invention being indicated by the appended claims and all changes which come Within the meaning and range of equivalency of the claims are intended to be embraced therein.

The invention claimed is:

1. In an ultrahigh vacuum system, the improvement comprising:

(a) a chamber enclosing structure including a honeycomb core panel having a thin inner wall, a thin outer wall and a core member provided with a plurality of perforations whereby the individual cells of said core member open into each other without materially affecting the structural strength of said honeycomb core panel;

(b) an ultrahigh vacuum pumping system connected to a first space enclosed by said inner wall; and

(c) a vacuum pumping system connected to a second space between said inner wall and said outer wall whereby the pressure differential across said inner wall is held to no greater than .010 mm. mercury thereby preventing the diffusion of gas through said inner wall into said first space as an ultrahigh vacuum condition is created within said first space.

2. The apparatus of claim 1 wherein the thickness of said inner wall is no greater than .010 inch.

3. The apparatus of claim 1 including additionally means for cooling said inner wall to prevent gas contained in said inner wall from diffusing into said first space as said ultrahigh vacuum condition is created within said first space.

4. The apparatus of claim 1 including additionally:

(a) a first means for heating said inner wall during the initial stages of evacuating said first and said second spaces to facilitate the diffusion of gas contained in said inner wall into said first and said second spaces; and

(b) a second means for subsequently cooling said inner Wall during the later stages of evacuating said first and said second spaces to prevent any residual gas remaining in said inner wall from diffusing into said first space as said ultrahigh vacuum condition is created within said first space.

5. The apparatus of claim 4 wherein the thickness of said inner wall is no greater than .010 inch.

6. An ultrahigh vacuum system comprising:

(a) a base;

(b) an enclosure mounted on said base, the wall structure of said enclosure comprising a honeycomb core panel having a thin inner wall, a thin outer wall and a core member provided with a plurality of perforations whereby the individual cells of said core member open into each other without materially affecting the structural strength of said honeycomb core panel;

(c) an inner O ring and an outer O ring effecting a double 0 ring seal between said enclosure and said base with a space between said 0 rings vented to a space between said inner wall and said outer Wall;

((1) an ultrahigh vacuum pumping system connected through said base to a chamber enclosed by said enclosure;

(e) a vacuum pumping system connected to said Wall structure so as to evacuate said space between said inner wall and said outer wall thereof and said space between said 0 rings whereby the pressure differential across said inner wall and said inner O ring, respectively, is held to a minimum thereby preventing the diffusion of gas through said inner wall and said inner O ring into said chamber as an ultrahigh vacuum condition is created within said chamber;

(f) a first means for heating said inner wall during the initial stages of evacuating said chamber and said spaces between said inner wall and said outer wall to facilitate the diffusion of gas contained in said inner wall into said chamber and said space between said inner wall and said outer wall;

(g) a second means for cooling said 0 rings simultaneously as said inner wall is heated by said first means to prevent heat damage to said 0 rings; and

(h) a third means for subsequently cooling said inner wall during the later stages of evacuating said chamber and said space between said inner wall and said outer wall to prevent any residual gas remaining in said inner wall from diffusing into said chamber as said ultrahigh vacuum condition is created within said chamber.

7. The apparatus of claim 6 wherein the thickness of said inner wall is no greater than .010 inch.

8. In an ultrahigh vacuum system, the improvement comprising:

(a) a chamber enclosure structure including an inner wall having a thickness no greater than .010 inch and a thin outer wall spacedly arranged with respect to each other and joined by an inner core structural member whereby said inner Wall, said outer Wall and said inner core structural member form an integral structural frame;

(b) an ultrahigh vacuum pumping system connecting to a first space enclosed by said inner wall; and

(c) a vacuum pumping system connected to a second space between said inner Wall and said outer wall whereby the pressure differential across the inner wall is held to no greater than .010 mm. mercury thereby preventing the diffusion of gas through said inner Wall into said first space as an ultrahigh vacuum condition is created within said first space.

9. The apparatus of claim 8 wherein said thin inner wall is formed of highly polished stainless steel.

References Cited by the Examiner UNITED STATES PATENTS 1,980,825 11/34 Rankin 13-31 2,177,233 10/39 Vincent 2209 X 2,415,425 2/47 Heineman 98-29 2,439,806 4/48 Heineman 2209 X 2,482,753 9/49 Heineman 98--29 X 2,770,932 11/56 Polye 53-6 3,018,561 1/62 Wells 9829 X 3,095,494 6/63 Denton et al. 219- FOREIGN PATENTS 219,036 12/58 Australia.

THERON E. CONDON, Primary Examiner. 

8. IN AN ULTRAHIGH VACUUM SYSTEM, THE IMPROVEMENT COMPRISING: (A) A CHAMBER ENCLOSURE STRUCTURE INCLUDING AN INNER WALL HAVING A THICKENSS NO GREATER THAN .010 INCH AND A THIN OUTER WALL SPACEDLY ARRANGED WITH RESPECT TO EACH OTHER AND JOINED BY AN INNER CORE STRUCTURAL MEMBER WHEREBY SAID INNER WALL, SAID OUTER WALL AND SAID INNER CORE STRUCTURAL MEMBER FORM AN INTEGRAL STRUCTURAL FRAME; (B) AN ULTRAHIGH VACUUM PUMPING SYSTEM CONNECTING TO A FIRST SPACE ENCLOSED BY SAID INNER WALL; AND (C) A VACUUM PUMPING SYSTEM CONNECTED TO A SECOND SPACE BETWEEN SAID INNER WALL AND SAID OUTER WALL WHEREBY THE PRESSURE DIFFERENTIAL ACROSS THE INNER WALL IS HELD TO NO GREATER THAN .010 MM. MERCURY THEREBY PREVENTING THE DIFFUSION OF GAS THROUGH SAID INNER WALL INTO SAID FIRST SPACE AS AN ULTRAHIGH VACUUM CONDITION IS CREATED WITHIN SAID FIRST SPACE. 